Aromatic polyester resins typified by polyethylene terephthalate and polybutylene terephthalate are inexpensive resins having both heat resistance and clarity, and if they are stretched to form films, they can be made to exhibit high mechanical strength. Therefore, they are used for various applications such as various industrial materials, packaging materials and optical materials.
On the other hand, in recent years, as means for obtaining polymers with higher functions and higher performance, polymer alloy techniques of alloying (adding/mixing) a main polymer with a dissimilar polymer are popularly used.
The reason is that while the properties required for polymers diversely include mechanical strength, light transmittance, gas barrier properties, heat resistance, moldability, releasability, flame retardancy, ultraviolet light resistance (UV resistance), hydrolysis resistance, electric insulation and the like, it is difficult that a single polymer is made to satisfy these diverse property requirements. That is, multiple polymers are alloyed with each other to achieve required diverse properties in increasing cases.
In this regard, aromatic polyester resins are excellent in mechanical strength, clarity (light transmittance) and heat resistance but are unsatisfactory in gas barrier properties, moldability, releasability, flame retardancy, UV resistance, hydrolysis resistance and electric insulation depending on applications. Therefore, efforts are being made to develop films improved in these properties by alloying with resins capable of covering these properties.
For example, there are a film obtained by alloying an aromatic polyester resin with a polycarbonate resin (JP 2004-107471 A), and a film obtained by alloying an aromatic polyester resin with a styrene-butadiene-styrene resin modified by an epoxy compound (JP 2003-075643 A). In the former case, mechanical strength and heat resistance are enhanced, and in the latter case, a film is monoaxially oriented to be improved in polarization properties.
However, even if the techniques described in JP 2004-107471 A and JP 2003-075643 A are used, there remains a problem that a film remarkably enhanced in gas barrier properties, moldability, releasability, UV resistance, electric insulation and the like cannot be obtained.
Polyolefin resins are more excellent than aromatic polyester resins in gas barrier properties, moldability, releasability, UV resistance, insulation and the like. Therefore, if a polyolefin resin is alloyed with an aromatic polyester resin, it is seemingly expected that a film excellent in gas barrier properties, moldability, releasability, UV resistance, electric insulation and the like can be obtained by using the alloy.
However, the bonding force between an aromatic polyester resin and a polyolefin resin is weak and, therefore, even if the mixture obtained by merely mixing both the resins is stretched to form a film, the aromatic polyester resin and the polyolefin resin are separated at the interfaces in the stretching step, to form many voids inside the film (for example, JP 6-322153 A).
If the voids are formed, the film is remarkably lowered in mechanical strength, moldability, gas barrier properties, light transmittance, insulation and productivity, far from being enhanced in these properties, and is not enhanced in releasability or UV resistance either.
It could therefore be helpful to provide a polyester film excellent in mechanical strength, light transmittance, gas barrier properties, moldability, releasability, UV transmittance, electric insulation and productivity.